JP6693607B1 - Hot rolled steel sheet and method of manufacturing the same - Google Patents

Abstract

A hot rolled steel sheet and a method for manufacturing the same are provided. The composition of the hot-rolled steel sheet of the present invention is, in mass%, C: 0.04% or more and 0.50% or less, Si: 2.0% or less, Mn: 0.5% or more and 3.0% or less, P. : 0.10% or less, S: 0.05% or less, Al: 0.005% or more and 0.10% or less, N: 0.010% or less, the balance being Fe and unavoidable impurities, and a steel sheet. The steel structure at the 1 / 2t position of the plate thickness t from the surface has a volume ratio of more than 30% ferrite and 10% or more bainite, and the total of ferrite and bainite is relative to the entire steel structure at the 1 / 2t position. 70% or more and 95% or less, the balance being one or two or more kinds selected from pearlite, martensite, and austenite, and crystallizing a region surrounded by a boundary in which the misorientation of adjacent crystals is 15 ° or more. In terms of grains, the average circle equivalent diameter of crystal grains is less than 7.0 μm, and the total of crystal grains having a circle equivalent diameter of 40.0 μm or more is the volume ratio with respect to the entire steel structure at the 1 / 2t position. Is 30% or less.

Description

  TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet having high strength and low yield ratio, which is suitable for a building structure member, and has excellent toughness, and a method for producing the hot-rolled steel sheet. The hot-rolled steel sheet of the present invention is particularly suitable for use as a raw material for a rectangular steel pipe manufactured by cold roll forming.

  2. Description of the Related Art In recent years, building structural members used for large-scale buildings (hereinafter referred to as buildings) such as factories, warehouses, and commercial facilities have been strengthened to reduce construction cost due to weight reduction. In particular, a rectangular steel pipe (square column) used as a pillar material of a building is required to have mechanical properties such as a yield strength of 385 MPa or more and a tensile strength of 520 MPa or more, and further has high plastic deformability from the viewpoint of earthquake resistance. At the same time, toughness is required. Therefore, it is necessary to appropriately select the material of the rectangular steel pipe.

  The rectangular steel pipe is generally manufactured by using a hot-rolled steel plate (hot-rolled steel strip) or a thick plate as a material and cold-forming the material. Examples of the cold forming method include a cold press bending method and a cold roll forming method. Usually, when manufacturing a square steel pipe by cold-roll forming a hot-rolled steel plate, first, the hot-rolled steel plate is formed into a round steel pipe, and then the round steel pipe is subjected to cold forming to obtain a square steel pipe. This method of manufacturing a rectangular steel pipe by roll forming has the advantages that it has higher productivity than the method of manufacturing a rectangular steel pipe by press bending, and that it can be manufactured in a short delivery time. However, in the method for manufacturing a square steel pipe by roll forming, particularly when forming a round steel pipe, a large processing strain is introduced in the pipe axial direction, so that the yield ratio in the pipe axial direction tends to increase and the toughness tends to decrease. There is a problem.

  Furthermore, in the roll-formed rectangular steel pipe, the greater the plate thickness, the greater the processing strain during roll forming, so the yield ratio becomes higher and the toughness becomes lower. Therefore, particularly when manufacturing a roll-formed square steel pipe having a thickness exceeding 20 mm, it is necessary to select a material that can withstand an increase in yield ratio and a decrease in toughness due to roll forming.

  In addition, as described above, for the material to be roll-formed, an appropriate hot-rolled steel sheet (hot-rolled steel strip) or thick plate is selected in consideration of changes in mechanical properties such as increase in yield ratio and decrease in toughness due to roll-forming. Required to do so.

  In order to meet such requirements, for example, in Patent Document 1, in weight%, C ≦ 0.02%, Si ≦ 1.0%, Mn: 0.05 to 2.0%, S ≦ 0.02% , Al: 0.01 to 0.1%, Nb: 0.08 to 0.25%, Ti ≦ 0.2%, B ≦ 0.0020%, and one of Ni, Cr, Sn, Cu Alternatively, the total content of two or more kinds is 0.02% or more and 0.3% or less, the balance is Fe and unavoidable impurities, and the amount of Nb is Nb ≧ 0.05 + 7.75C-1.98Ti + 6.64N + 0.000035 / (B + 0.0004), the ferrite has a volume fraction of 70% or more, the ferrite grain size is 10.5 or more and 15 or less in grain size, and the yield ratio at room temperature is 70%. A low yield ratio fire-resistant hot-rolled steel sheet excellent in toughness is disclosed by the following. .

  In Patent Document 2, C: 0.07 to 0.18%, Mn: 0.3 to 1.5%, P: 0.03% or less, S: 0.015% or less, and Al: 0.01 to 0.06%, N: 0.006% or less, a composition consisting of the balance Fe and unavoidable impurities, ferrite as a main phase, and pearlite or pearlite and bainite as a second phase. However, the second phase frequency defined by a predetermined formula is 0.20 to 0.42, and the structure has an average crystal grain size including the main phase and the second phase of 7 to 15 μm, which results in toughness. A thick-walled hot-rolled steel sheet for square steel pipes for building structural members is disclosed.

In Patent Document 3, C: 0.06 to 0.12% (meaning mass%, the same applies hereinafter), Si: 0.05 to 0.5%, Mn: 1.0 to 1.8%, Al: 0.01-0.06%, P: 0.025% or less (0% is not included), S: 0.01% or less (0% is not included), Nb: 0.005-0.025%, Ti: 0.005-0.03%, N: 0.002-0.009%, and B: 0.0005-0.003% are contained, respectively, and the carbon equivalent Ceq defined by a predetermined formula is 0. .40% or less, the balance is composed of iron and unavoidable impurities, and the structure is mainly composed of a bainite phase, and they are adjacent to each other at a depth t / 4 (t represents a plate thickness, the same applies hereinafter) from the surface. When a region surrounded by large-angle grain boundaries with a crystal orientation difference of 15 ° or more is defined as a crystal grain, the crystal grain is subjected to electron backscattering diffraction. The average equivalent circle diameter D _A measured by the image method is 10 μm or less, and the grain size of the crystal grains measured by the electron backscattering diffraction image method is the predicted maximum calculated by the extreme value statistical method based on a predetermined formula. By setting the grain size D _M to 80 μm or less, a high-strength steel plate for large heat input welding, which is excellent in base material low temperature toughness, is disclosed.

  Patent Document 4 contains C: 0.04 to 0.25%, N: 0.0050 to 0.0150%, and Ti: 0.003 to 0.050% by weight, and calculates by a predetermined formula. The carbon equivalent (Ceq.) Is 0.10 to 0.45%, the pearlite phase is in the range of 5 to 20% in area fraction, and the average grain size is 1 to 1 in the steel. A high-strength hot-rolled steel sheet excellent in uniform elongation after cold working (that is, having a low yield ratio) is disclosed by dispersing 30 μm of TiN in a proportion of 0.0008 to 0.015% by weight. There is.

In Patent Document 5, the carbon equivalent Ceq calculated from the steel component (mass%) is 0.33% or more and 0.43% or less, the weld crack susceptibility composition P _CM is 0.15% or more and 0.24% or less, and welding Disclosed is a thick plate for cold press-formed square steel pipe, which is made of steel having a composition having a heat-affected zone toughness index f _HAZ of 0.30% or more and 0.47% or less. In the thick steel plate for cold press-formed square steel pipe of Patent Document 5, the steel structure is composed of ferrite and the balance bainite or pearlite.

In Patent Document 6, C: 0.05 to 0.20%, Si: 0.10 to 0.40%, Mn: 1.20 to 1.50%, and Al: 0.003 to 0 in mass%. Steel material containing 0.06%, Ti: 0.005 to 0.050%, the balance being Fe and impurities, and having a Ceq defined by the following formula of 0.34 or more, heated to 900 to 1200 ° C. after starts rolling, after completion of rolling in Ar ₃ point or more, and water cooling from below the Ar ₃ point to Ar ₃ point -400 ° C. or less, then the manufacturing method of square steel for steel sheets for tempering at 500 ° C. or less It is disclosed. The steel sheet for rectangular steel pipe of Patent Document 6 has a steel structure composed of soft ferrite and hard bainite or martensite.

Japanese Patent No. 4276324 Japanese Patent No. 5589885 Japanese Patent No. 5096087 JP-A-7-224351 JP, 2016-11439, A Japanese Patent No. 5655525

  However, in the technique of Patent Document 1, the C content, which is an element that greatly contributes to the strengthening of steel, is suppressed to 0.02 wt% or less. Therefore, there is a problem that it is difficult to stably set the yield strength after roll forming to 385 MPa or more.

  In the technique of Patent Document 2, the average crystal grain size including the main phase and the second phase is 7 to 15 μm. In the range of the average crystal grain size, there is a problem that a tensile strength of 520 MPa or more cannot be obtained after roll forming.

  In the technique of Patent Document 3, the bainite phase is the main component (70 area% or more). Since the area ratio of hard bainite is high, there is a problem that the yield ratio exceeds 0.75.

  The technique of Patent Document 4 is a composite microstructure steel of soft ferrite and hard pearlite. Therefore, the yield ratio is low, but the toughness is poor, and there is a problem that the toughness required for the rectangular steel pipe cannot be secured.

  When the thick plate for cold press forming square steel pipe obtained by the technique of Patent Document 5 is applied to the material of the cold roll forming square steel plate, the toughness is deteriorated due to the processing strain introduced in the pipe axial direction during the cold roll forming. descend. Therefore, there is a problem that the toughness necessary for the rectangular steel pipe cannot be secured.

  The steel sheet manufactured by the above manufacturing method of Patent Document 6 requires a tempering treatment after hot rolling and subsequent cooling in order to set the yield ratio to 0.75 or less. Therefore, it is disadvantageous in terms of manufacturing cost.

  The present invention has been made in view of the above circumstances, and provides a hot-rolled steel sheet having a high strength and a low yield ratio, which is suitable for a building structure member, and is excellent in toughness, and a manufacturing method thereof. With the goal.

  Here, "high strength" in the present invention means a hot-rolled steel sheet (square steel pipe) which is a raw material of a square steel pipe manufactured by cold roll forming (hereinafter, sometimes referred to as cold roll forming square steel pipe). Hot rolled steel sheet) has a yield strength of 330 MPa or more and a tensile strength of 520 MPa or more. The "low yield ratio" in the present invention means that the yield ratio (= yield strength / tensile strength) of the above material is 0.75 or less. The term “excellent in toughness” as used in the present invention means that the Charpy absorbed energy at −40 ° C. of the above material is 170 J or more. The term "thick" in the present invention means that the plate thickness is more than 20 mm. In the present invention, the hot-rolled steel sheet of the above material includes a hot-rolled steel strip.

  The present inventors have diligently studied to solve the above problems.

  As described above, the material to be roll-formed needs to be appropriately selected in consideration of changes in mechanical properties such as increase in yield ratio and decrease in toughness due to roll-forming. In the present invention, first, a square steel pipe manufactured by cold-roll forming a material is examined for a material capable of having a yield strength of 385 MPa or more, a tensile strength of 520 MPa or more, and high plastic deformability and excellent toughness. did. As a result, the mechanical properties of the material for cold roll forming square steel pipe (hot rolled steel sheet) are as follows: yield strength is 330 MPa or more, tensile strength is 520 MPa or more, and yield ratio (= yield strength / tensile strength) is 0. It has been found that the Charpy absorbed energy at 0.75 or less and at -40 ° C is 170 J or more.

Then, as a result of further studying a material (hot rolled steel sheet for cold roll forming square steel tube) satisfying the above mechanical characteristics, the following findings (i) to (iii) were obtained.
(I) In order for the material to satisfy the yield strength and tensile strength targeted in the present invention, the C content is 0.04 mass% or more, and the main structure of the steel sheet is a mixed structure of ferrite and bainite, Also, when the region surrounded by the boundary where the crystal orientation difference between adjacent crystals is 15 ° or more is a crystal grain, it is necessary to set the average equivalent circle diameter of the crystal grain to less than 7.0 μm.
(Ii) In order for the material to satisfy the yield ratio targeted in the present invention, it is necessary that the balance structure of the steel sheet is one or more selected from hard pearlite, martensite, and austenite. is there.
(Iii) In a steel structure in which the material satisfies both (i) and (ii) described above, in order to further have the toughness that is the object of the present invention, it is surrounded by a boundary having an orientation difference of 15 ° or more between adjacent crystals. When the region to be formed is a crystal grain, the volume ratio of the crystal grain having a circle equivalent diameter of 40.0 μm or more needs to be 30% or less.

The present invention has been completed based on these findings and has the following gist.
[1] The composition of components is% by mass,
C: 0.04% or more and 0.50% or less,
Si: 2.0% or less,
Mn: 0.5% to 3.0%,
P: 0.10% or less,
S: 0.05% or less,
Al: 0.005% or more and 0.10% or less,
N: 0.010% or less is contained, and the balance is Fe and inevitable impurities,
The steel structure at the 1 / 2t position of the plate thickness t from the surface of the steel plate is
In volume ratio, ferrite is more than 30%, bainite is 10% or more,
The total of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at the 1 / 2t position,
The balance consists of one or more selected from pearlite, martensite and austenite,
When the region surrounded by the boundary where the misorientation of adjacent crystals is 15 ° or more is a crystal grain,
The average equivalent circular diameter of the crystal grains is less than 7.0 μm,
Further, the hot-rolled steel sheet, wherein the total of the crystal grains having a circle equivalent diameter of 40.0 μm or more is 30% or less in terms of volume ratio with respect to the entire steel structure at the 1 / 2t position.
[2] The hot-rolled steel sheet according to the above [1], which further contains, in mass%, one group or two groups selected from the following Group A and Group B in addition to the above component composition.
Note One or more selected from the group A: Nb: 0.15% or less, Ti: 0.15% or less, V: 0.15% or less, and the group B: Cr: 1.0% or less, One or two selected from Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.010% or less. [3] The hot-rolled steel sheet according to the above [1] or [2], which has a thickness of more than 20 mm.
[4] After heating the steel material having the component composition described in [1] or [2] above to a heating temperature of 1100 ° C. or higher and 1300 ° C. or lower,
Rough rolling end temperature: 850 ° C. or more and 1150 ° C. or less, finish rolling end temperature: 750 ° C. or more and 850 ° C. or less, and total rolling reduction at 930 ° C. or less: 65% or more, hot rolling is performed,
After the hot rolling, a method for producing a hot-rolled steel sheet, comprising cooling at an average cooling rate at a sheet thickness center temperature: 10 ° C / s or more and 30 ° C / s or less and a cooling stop temperature: 450 ° C or more and 650 ° C or less.
[5] The method for manufacturing a hot rolled steel sheet according to the above [4], wherein the hot rolled steel sheet has a thickness of more than 20 mm.

  According to the present invention, it has a high strength and a low yield ratio and is excellent in toughness, that is, the yield strength is 330 MPa or more, the tensile strength is 520 MPa or more, the yield ratio is 0.75 or less, and the Charpy absorbed energy at −40 ° C. is 170 J. It is possible to provide the hot-rolled steel sheet and the manufacturing method thereof that are as described above.

  Hereinafter, the present invention will be described in detail.

  The hot rolled steel sheet of the present invention is, in mass%, C: 0.04% or more and 0.50% or less, Si: 2.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0. 10% or less, S: 0.05% or less, Al: 0.005% or more and 0.10% or less, N: 0.010% or less, with the balance being Fe and inevitable impurities. The steel structure at the 1 / 2t position of the plate thickness t from the steel plate surface has a volume ratio of more than 30% ferrite and 10% or more bainite, and the total of the ferrite and the bainite is the steel structure at the 1 / 2t position. It is 70% or more and 95% or less with respect to the whole, and the balance consists of one or more selected from pearlite, martensite, and austenite. In addition, when a region surrounded by a boundary in which the misorientation of adjacent crystals (hereinafter, also referred to as “crystal misorientation”) is 15 ° or more is defined as a crystal grain, the average equivalent circle diameter of the crystal grain (hereinafter, “average The steel structure at the 1 / 2t position is the total of the crystal grains having an equivalent circle diameter (hereinafter, also referred to as “crystal grain size”) of 40.0 μm or more, which is less than 7.0 μm. The volume ratio is 30% or less with respect to the whole.

  First, the reason why the composition of the steel material of the hot rolled steel sheet is limited in the present invention will be described below. In the present specification, unless otherwise specified, “%” indicating the steel composition is “mass%”.

C: 0.04% or more and 0.50% or less C is an element that increases the strength of steel by solid solution strengthening. Further, C is an element that promotes the formation of pearlite, enhances the hardenability, contributes to the production of martensite, and contributes to the stabilization of austenite, and thus contributes to the formation of the hard phase. In order to secure the strength and yield ratio targeted by the present invention, it is necessary to contain C in an amount of 0.04% or more. However, if the C content exceeds 0.50%, the ratio of the hard phase increases, the yield ratio increases, the toughness decreases, and the weldability deteriorates. Therefore, the C content is 0.04% or more and 0.50% or less. The C content is preferably 0.08% or more, more preferably more than 0.12%, and even more preferably 0.14% or more. Further, the C content is preferably 0.30% or less, more preferably 0.25% or less, and even more preferably 0.22% or less.

Si: 2.0% or less Si is an element that increases the strength of steel by solid solution strengthening, and can be contained if necessary. In order to obtain such an effect, it is desirable that Si is contained by 0.01% or more. However, if the Si content exceeds 2.0%, the weldability deteriorates. Also, the toughness is reduced. Therefore, the Si content is 2.0% or less. The Si content is preferably 0.01% or more, more preferably 0.10% or more. The Si content is preferably 0.5% or less, more preferably 0.4% or less, and even more preferably 0.3% or less.

Mn: 0.5% to 3.0% Mn is an element that increases the strength of steel by solid solution strengthening. Mn is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. In order to secure the desired strength and structure in the present invention, Mn needs to be contained in an amount of 0.5% or more. However, if the Mn content exceeds 3.0%, the weldability deteriorates. Further, the yield strength becomes high and the desired yield ratio cannot be obtained. Therefore, the Mn content is 0.5% or more and 3.0% or less. The Mn content is preferably 0.7% or more, more preferably 0.9% or more, still more preferably 1.0% or more. Further, the Mn content is preferably 2.5% or less, more preferably 2.0% or less.

P: 0.10% or less P segregates at grain boundaries and causes inhomogeneity of the material, so it is preferable to reduce P as inevitable impurities as much as possible, but a content of 0.10% or less is acceptable. Therefore, the P content is within the range of 0.10% or less. The P content is preferably 0.03% or less, more preferably 0.020% or less, and even more preferably 0.015% or less. Although the lower limit of P is not particularly specified, excessive reduction causes a rise in smelting cost, so P is preferably 0.002% or more.

S: 0.05% or less S is usually present as MnS in steel, but MnS is thinly stretched in the hot rolling step and adversely affects ductility. Therefore, in the present invention, it is preferable to reduce S as much as possible, but a content of 0.05% or less is acceptable. Therefore, the S content is 0.05% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less, and even more preferably 0.008% or less. In addition, although the lower limit of S is not particularly specified, excessive reduction causes a rise in smelting cost, so S is preferably 0.0002% or more.

Al: 0.005% or more and 0.10% or less Al is an element that acts as a strong deoxidizing agent. In order to obtain such an effect, it is necessary to contain Al in an amount of 0.005% or more. However, when the Al content exceeds 0.10%, the weldability deteriorates, and the alumina-based inclusions increase, so that the surface quality deteriorates. In addition, the toughness of the welded part is also reduced. Therefore, the Al content is 0.005% or more and 0.10% or less. The Al content is preferably 0.01% or more, more preferably 0.027% or more. Further, the Al content is preferably 0.07% or less, more preferably 0.04% or less.

N: 0.010% or less N is an unavoidable impurity and is an element having a function of lowering toughness by firmly fixing dislocation motion. In the present invention, it is desirable to reduce N as an impurity as much as possible, but the content of N can be allowed up to 0.010%. Therefore, the N content is 0.010% or less. The N content is preferably 0.0080% or less, more preferably 0.0040% or less, and even more preferably 0.0035% or less. In addition, since excessive reduction leads to an increase in smelting cost, the N content is preferably 0.0010% or more, and more preferably 0.0015% or more.

  The balance is Fe and inevitable impurities. However, as long as the effect of the present invention is not impaired, it does not mean that O is contained in an amount of 0.005% or less.

  The above components are the basic component compositions of the hot rolled steel sheet according to the present invention. Although the above-mentioned essential elements can provide the characteristics desired in the present invention, the following elements can be contained if necessary.

One or more selected from Nb: 0.15% or less, Ti: 0.15% or less, V: 0.15% or less Nb, Ti, and V are all fine carbides in steel. , Is an element that forms a nitride and contributes to the strength improvement of steel through precipitation strengthening, and can be contained if necessary. In order to obtain such an effect, when Nb, Ti, and V are contained, it is preferable that Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more, respectively. On the other hand, an excessive content may cause an increase in yield ratio and a decrease in toughness. Therefore, when Nb, Ti, and V are contained, it is preferable that Nb: 0.15% or less, Ti: 0.15% or less, and V: 0.15% or less, respectively. For this reason, when Nb, Ti, and V are contained, it is preferable that Nb: 0.15% or less, Ti: 0.15% or less, and V: 0.15% or less, respectively. It is preferable that Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more. More preferably, Nb: 0.008% or more and 0.10% or less, Ti: 0.008% or more and 0.10% or less, and V: 0.008% or more and 0.10% or less. Even more preferably, Nb: 0.010% or more and 0.035% or less, Ti: 0.010% or more and 0.040% or less, V: 0.010% or more and 0.035% or less. When two or more kinds selected from Nb, Ti and V are contained, the yield ratio may be increased and the toughness may be decreased. Therefore, the total amount (the amount of Nb + Ti + V) is set to 0.150% or less. Preferably.

Cr: 1.0% or less, Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.010% or less One or more selected types Cr: 1.0% or less, Mo: 1.0% or less Cr and Mo are elements that enhance the hardenability of steel and increase the strength of steel, and if necessary. Can be included. In order to obtain the above-mentioned effects, when Cr and Mo are contained, it is preferable that Cr: 0.01% or more and Mo: 0.01% or more, respectively. On the other hand, an excessive content may lead to a decrease in toughness and a deterioration in weldability. Therefore, when Cr and Mo are contained, it is preferable that Cr: 1.0% or less and Mo: 1.0% or less, respectively. Therefore, when Cr and Mo are contained, it is preferable that Cr: 1.0% or less and Mo: 1.0% or less, respectively. In addition, it is preferable that Cr: 0.01% or more and Mo: 0.01% or more.
More preferably, Cr: 0.10% or more and 0.50% or less and Mo: 0.10% or more and 0.50% or less.

Cu: 0.5% or less, Ni: 0.3% or less Cu and Ni are elements that increase the strength of steel by solid solution strengthening, and can be contained if necessary. In order to obtain the above-mentioned effects, when Cu and Ni are contained, it is preferable that Cu: 0.01% or more and Ni: 0.01% or more, respectively. On the other hand, an excessive content may lead to a decrease in toughness and a deterioration in weldability. Therefore, when Cu and Ni are contained, it is preferable that Cu: 0.5% or less and Ni: 0.3% or less, respectively. Therefore, when Cu and Ni are contained, it is preferable that the Cu content is 0.5% or less and the Ni content is 0.3% or less. In addition, it is preferable that Cu: 0.01% or more and Ni: 0.01% or more. More preferably, Cu: 0.10% or more and 0.4% or less and Ni: 0.10% or more and 0.2% or less.

Ca: 0.010% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling step, and may be contained if necessary. it can. In order to obtain such effects, when Ca is contained, it is preferably contained in 0.0005% or more. However, when the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel and the toughness may deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.010% or less. The Ca content is preferably 0.0005% or more. More preferably, the Ca content is 0.0010% or more and 0.0050% or less.

B: 0.010% or less B is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. In order to obtain such an effect, when B is contained, it is preferably contained in an amount of 0.0003% or more. However, if the B content exceeds 0.010%, the yield ratio may increase. Therefore, when B is contained, it is preferably 0.010% or less. Note that it is preferably 0.0003% or more.
More preferably, the B content is 0.0005% or more and 0.0050% or less.

  Next, the reason for limiting the steel structure of the hot rolled steel sheet according to the present invention will be described.

  In the hot-rolled steel sheet of the present invention, the steel structure at the sheet thickness: 1 / 2t position (t represents the sheet thickness, the same applies hereinafter) has a volume ratio of ferrite of more than 30% and bainite of 10% or more. And the total of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at the 1 / 2t position, and the balance is one or two selected from pearlite, martensite, and austenite. It consists of the above. When the region surrounded by the boundaries in which the orientation difference between adjacent crystals is 15 ° or more is a crystal grain, the average equivalent circle diameter (average crystal grain size) of the crystal grains is less than 7.0 μm, and the circle equivalent The total of the crystal grains having a diameter (crystal grain size) of 40.0 μm or more is 30% or less in volume ratio with respect to the entire steel structure at the 1 / 2t position.

Volume ratio of ferrite: more than 30%, volume ratio of bainite: 10% or more, total volume ratio of ferrite and bainite to the entire steel structure: 70% or more and 95% or less Ferrite is a soft structure and other hard structures. A low yield ratio can be realized by mixing with. In order to obtain the low yield ratio targeted by the present invention due to such effects, the volume fraction of ferrite needs to exceed 30%. The volume ratio of ferrite is preferably 40% or more, more preferably 43% or more, and even more preferably 45% or more. The upper limit is not particularly specified, but in order to secure a desired yield ratio, the volume fraction of ferrite is preferably less than 75%, more preferably less than 70%, and even more preferably 60% or less. ..

  Bainite is a structure having an intermediate hardness and increases the strength of steel. Since the yield strength and the tensile strength targeted by the present invention cannot be obtained only by the above-mentioned ferrite, it is necessary to set the volume ratio of bainite to 10% or more. The volume ratio of bainite is preferably 15% or more, more preferably 20% or more, still more preferably 25% or more. The upper limit is not particularly specified, but in order to secure a desired yield ratio, the volume ratio of bainite is preferably 55% or less, more preferably 50% or less, and even more preferably 45% or less. ..

  However, if the total volume ratio of ferrite and bainite is less than 70%, the yield ratio and toughness targeted by the present invention cannot be obtained. On the other hand, if the total volume fraction of ferrite and bainite exceeds 95%, the yield strength and yield ratio targeted by the present invention cannot be obtained. Therefore, in addition to the above-mentioned conditions, it is necessary to make the total volume ratio of ferrite and bainite 70% or more and 95% or less. It is preferably 75% or more and 93% or less. More preferably, it is 80% or more and 90% or less.

Remainder: one or more selected from pearlite, martensite, and austenite Pearlite, martensite, and austenite have a hard structure. By increasing the tensile strength of steel and mixing with soft ferrite, A low yield ratio can be realized. In order to obtain such an effect, it is preferable that the total volume ratio of pearlite, martensite, and austenite be 5% or more and 30% or less. More preferably, it is 7% or more and 25% or less. Even more preferably, it is 10% or more and 20% or less. The volume ratios of ferrite, bainite, pearlite, martensite, and austenite can be measured by the method described in Examples below.

When the region surrounded by the boundaries in which the difference in crystal orientation between adjacent crystals (crystal orientation difference) is 15 ° or more is defined as a crystal grain, the average crystal grain size of the crystal grain is less than 7.0 μm, and the crystal grain size is 40.0 μm or more. The total volume ratio of the crystal grains of: 30% or less As described above, the steel structure of the present invention has a soft structure and a hard structure in order to obtain the low yield ratio, the yield strength, and the tensile strength aimed at by the present invention. A mixed steel (hereinafter referred to as "composite steel"). However, the composite structure steel has poor toughness as compared with the single structure steel. Therefore, in the present invention, in order to achieve both the mechanical properties and the excellent toughness, the average crystal grain size of the crystal grains is defined when the region surrounded by the boundary having the crystal orientation difference of 15 ° or more is defined as the crystal grain. When the average crystal grain size of the crystal grains is 7.0 μm or more, the ferrite grains are not sufficiently fine, and the desired yield strength and toughness cannot be obtained. Therefore, by setting the average crystal grain size of the crystal grains to less than 7.0 μm, the yield strength targeted in the present invention is obtained and the toughness is secured. The average crystal grain size of the crystal grains is preferably 6.5 μm or less, and more preferably 6.0 μm or less.

  In general, the grain size distribution in a single-structure steel or a steel close to a single-structure steel follows a normal logarithmic distribution that has one peak, spreads largely on the variable side, and is limited on the variable side. However, it was found that a bainite peak newly appears on the coarse grain side in the crystal grain size distribution in the composite structure steel containing ferrite and bainite as in the present invention.

  Specifically, in the steel structure of the present invention, that is, in the composite structure steel in which the volume ratio of ferrite exceeds 30% and the volume ratio of bainite is 10% or more, the bainite peak is newly added to the coarse grain side in the grain size distribution. Appears in. This indicates that coarse bainite is mixed. The mixture of coarse bainite causes a great deterioration in toughness. As a result, in the composite steel, even if the upper limit of the maximum crystal grain size is specified, the proportion of coarse bainite present cannot be kept low. Therefore, in order to obtain good toughness, it is also necessary to specify the upper limit of the proportion of coarse crystal grains.

  Bainite does not grow beyond boundaries with large misorientations (austenite grain boundaries and sub-boundaries formed by dislocation accumulation). In order to suppress the formation of the above coarse bainite, finish rolling in hot rolling is performed at a temperature as low as possible, and a large amount of dislocations are introduced into austenite to increase the subboundary area to form a fine subgrain structure. (Hereinafter, also referred to as “miniaturization”) is particularly effective.

  That is, the toughness of the steel in the present invention is improved by increasing the total area of the grain boundaries which is the resistance to brittle fracture. By preliminary experiments, it was newly found that if the volume ratio of coarse crystal grains having a crystal grain size of 40.0 μm or more exceeds 30%, it is not possible to secure a sufficient grain boundary area to obtain the required toughness. Therefore, in the present invention, in addition to defining the upper limit of the average crystal grain size of the crystal grains to be less than 7.0 μm, the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more is 30% or less. Stipulates that The volume ratio of crystal grains having a crystal grain size of 40.0 μm or more is preferably 20% or less, more preferably 15% or less.

  The crystal orientation difference, the average crystal grain size, and the volume ratio of the crystal grains having a crystal grain size of 40.0 μm or more can be measured by the SEM / EBSD method, and are described in Examples described later herein. Can be measured by the method.

  Next, a method for manufacturing the hot-rolled steel sheet according to the embodiment of the present invention will be described.

  The hot-rolled steel sheet of the present invention, for example, after heating a steel material having the above-described composition to a heating temperature of 1100 ° C or higher and 1300 ° C or lower, a rough rolling end temperature: 850 ° C or higher and 1150 ° C or lower, a finish rolling end temperature. : 750 ° C or more and 850 ° C or less, and 930 ° C or less, and a total reduction rate up to the finishing rolling end temperature: 65% or more, hot rolling is performed to obtain a hot rolled sheet, and after the hot rolling, a sheet is formed on the hot rolled sheet. By performing cooling at an average cooling rate of 10 ° C./s or more and 30 ° C./s or less at a thick center temperature, a cooling stop temperature: 450 ° C. or more and 650 ° C. or less, and performing a winding step of winding the hot-rolled sheet after cooling. can get.

  In the following description of the manufacturing method, “° C.” regarding temperature is the surface temperature of a steel material or a steel sheet (hot rolled sheet) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. The temperature at the center of the steel plate thickness can be obtained by calculating the temperature distribution in the steel plate cross section by heat transfer analysis and correcting the result with the surface temperature of the steel plate.

  In the present invention, the melting method of the steel material (steel slab) is not particularly limited, and any known melting method such as a converter, an electric furnace and a vacuum melting furnace is suitable. The casting method is also not particularly limited, but the desired dimension is produced by a known casting method such as a continuous casting method. It should be noted that there is no problem even if the ingot-bulk rolling method is applied instead of the continuous casting method. The molten steel may be further subjected to secondary refining such as ladle refining.

  Next, after heating the obtained steel material (steel slab) to a heating temperature of 1100 ° C. or more and 1300 ° C. or less, rough rolling ending temperature: 850 ° C. or more and 1150 ° C. or less is performed, and finish rolling ending temperature: Finish rolling is performed at 750 ° C. or more and 850 ° C. or less, and a hot rolling process is performed at a total reduction ratio of 930 ° C. or less: 65% or more to obtain a hot rolled sheet.

Heating temperature: 1100 ° C. or more and 1300 ° C. or less When the heating temperature is less than 1100 ° C., the deformation resistance of the material to be rolled increases and rolling becomes difficult. On the other hand, if the heating temperature exceeds 1300 ° C., the austenite grains become coarse, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), and the average crystal grain of the hot-rolled steel sheet aimed at by the present invention is obtained. It becomes difficult to secure the diameter. Further, it becomes difficult to suppress the formation of coarse bainite, and it is difficult to control the volume ratio of the crystal grains having a crystal grain size of 40.0 μm or more within the target range of the present invention. Therefore, the heating temperature in the hot rolling step is set to 1100 ° C or higher and 1300 ° C or lower. More preferably, it is 1120 ° C or higher and 1280 ° C or lower.

  In the present invention, after manufacturing a steel slab (slab), in addition to the conventional method of once cooling to room temperature and then heating again, charging into a heating furnace as it is without cooling to room temperature, or The energy-saving process of these direct feed rolling, in which rolling is performed immediately after a slight heat retention, can be applied without any problem.

Rough rolling end temperature: 850 ° C. or more and 1150 ° C. or less When the rough rolling end temperature is less than 850 ° C., the steel sheet surface temperature becomes equal to or lower than the ferrite transformation start temperature during the subsequent finish rolling, and the risk of ferrite formation increases. .. The generated ferrite becomes processed ferrite grains elongated in the rolling direction by the subsequent finish rolling, which causes an increase in the yield ratio. On the other hand, when the rough rolling finish temperature exceeds 1150 ° C, the amount of reduction in the austenite unrecrystallized temperature range is insufficient, and fine austenite grains cannot be obtained, and the average crystal grain size of the hot-rolled steel sheet of the present invention is obtained. Will be difficult to secure. Moreover, it becomes difficult to suppress the formation of coarse bainite. Therefore, the rough rolling end temperature is set to 850 ° C. or higher and 1150 ° C. or lower. More preferably, it is 860 ° C or higher and 1000 ° C or lower. More preferably, it is 870 ° C or higher and 980 ° C or lower.

Finishing rolling end temperature: 750 ° C. or more and 850 ° C. or less When the finishing rolling end temperature is less than 750 ° C., the steel sheet surface temperature becomes equal to or lower than the ferrite transformation start temperature during finish rolling, and the risk of ferrite formation increases. The ferrite generated as described above becomes worked ferrite grains elongated in the rolling direction by the subsequent rolling, which causes an increase in the yield ratio. On the other hand, when the finish rolling finish temperature exceeds 850 ° C., the amount of reduction in the austenite unrecrystallized temperature range is insufficient, and fine austenite grains cannot be obtained, and the average crystal grain size of the hot-rolled steel sheet of the present invention is obtained. Will be difficult to secure. Moreover, it becomes difficult to suppress the formation of coarse bainite. Therefore, the finish rolling end temperature is set to 750 ° C. or higher and 850 ° C. or lower. More preferably, it is 770 ° C or higher and 830 ° C or lower. More preferably, it is 780 ° C. or higher and 820 ° C. or lower.

Total rolling reduction at 930 ° C. or lower: 65% or higher In the present invention, by refining austenite in the hot rolling step, ferrite, bainite and the balance structure produced in the subsequent cooling step and winding step are refined, A hot-rolled steel sheet having the strength and toughness targeted by the invention can be obtained. In order to refine austenite in the hot rolling process, it is necessary to increase the rolling reduction in the austenite non-recrystallization temperature range and to introduce a sufficient processing strain. In order to achieve this, in the present invention, the total reduction ratio up to the finish rolling end temperature of 930 ° C. or less is set to 65% or more. When the total reduction ratio up to the finish rolling end temperature of 930 ° C. or less is less than 65%, sufficient working strain cannot be introduced in the hot rolling process, so that the structure having the target crystal grain size in the present invention is I can't get it. The total reduction ratio up to the finish rolling finishing temperature of 930 ° C. or lower is more preferably 70% or more, and further preferably 71% or more. The upper limit is not particularly specified, but if it exceeds 80%, the effect of improving the toughness with respect to an increase in the rolling reduction becomes small, and the equipment load only increases. Therefore, the total rolling reduction up to the finish rolling finish temperature is 930 ° C or less and 80% or less. Is preferred. It is more preferably 75% or less, and even more preferably 74% or less.

  Here, the reason why the temperature is 930 ° C. or lower is that if the temperature exceeds 930 ° C., the austenite is recrystallized in the rolling process and the dislocations introduced by the rolling disappear, so that a fine austenite cannot be obtained. The above-mentioned total reduction rate refers to the total reduction rate of each rolling pass in the temperature range up to 930 ° C. or lower until the finish rolling end temperature.

  In the hot rolling of the slab, the hot rolling may be performed in both the rough rolling and the finish rolling described above so that the total reduction ratio to the finish rolling end temperature is 930 ° C. or less and 65% or more, or only the finish rolling. It is also possible to perform hot rolling in which the total reduction rate up to the finish rolling end temperature of 930 ° C. or less is 65% or more. In the latter case, if the total rolling reduction to the finish rolling end temperature cannot be set to 65% or more by finish rolling alone, the slab is cooled in the course of rough rolling to a temperature of 930 ° C. or less. The total reduction rate up to the finish rolling end temperature of 930 ° C. or less in both rough rolling and finish rolling may be 65% or more.

  In the present invention, the upper limit of the finish plate thickness is not particularly specified, but it is preferably 32 mm or less from the viewpoint of ensuring the necessary reduction ratio and controlling the steel plate temperature.

  After the hot rolling step, the hot rolled sheet is cooled. In the cooling step, cooling is performed at an average cooling rate up to the cooling stop temperature: 10 ° C./s or more and 30 ° C./s or less, and a cooling stop temperature: 450 ° C. or more and 650 ° C. or less.

Average cooling rate from the start of cooling to the stop of cooling (end of cooling): 10 ° C / s or more and 30 ° C / s or less The average cooling rate in the temperature range from the start of cooling to the stop of cooling described below at the center temperature of the hot rolled sheet However, if it is less than 10 ° C./s, the nucleation frequency of ferrite decreases and the ferrite grains become coarse, so that the average crystal grain size cannot be less than 7.0 μm. Further, it is difficult to control the crystal grain size aimed at in the present invention within the range of the volume ratio of 40.0 μm or more. On the other hand, when the average cooling rate exceeds 30 ° C./s, a large amount of martensite is generated at the position of the plate thickness t / 2 of the steel sheet, and the total volume ratio of ferrite and bainite becomes less than 70%. The average cooling rate is preferably 15 ° C./s or more, more preferably 17 ° C./s or more. It is preferably 25 ° C./s or less, more preferably 23 ° C./s or less.

  In the present invention, from the viewpoint of suppressing the formation of ferrite on the surface of the steel sheet before cooling, it is preferable to start cooling immediately after finishing rolling.

Cooling stop temperature: 450 ° C or more and 650 ° C or less At the plate thickness center temperature of the hot-rolled sheet, and when the cooling stop temperature is less than 450 ° C, a large amount of martensite is generated at the sheet thickness 1 / 2t position of the steel sheet, and ferrite and bainite are formed. The total volume ratio may be less than 70%. Further, the volume ratio of ferrite may be 30% or less. On the other hand, when the cooling stop temperature exceeds 650 ° C., the nucleation frequency of ferrite decreases, the ferrite grains coarsen, and since the bainite transformation start temperature is exceeded, the volume ratio of bainite cannot be 10% or more. .. The cooling stop temperature is preferably 480 ° C. or higher, more preferably 490 ° C. or higher. The temperature is preferably 620 ° C or lower, and more preferably 600 ° C or lower.

  In the present invention, unless otherwise specified, the average cooling rate is a value obtained by ((center temperature of hot-rolled sheet before cooling-center temperature of hot-rolled sheet after cooling) / cooling time) (Cooling rate). Examples of the cooling method include water cooling such as jetting of water from a nozzle, cooling by jetting a cooling gas, and the like. In the present invention, it is preferable to perform a cooling operation (treatment) on both surfaces of the hot rolled sheet so that both surfaces of the hot rolled sheet are cooled under the same conditions.

After the cooling step, the hot rolled sheet is wound up, and then a winding step of allowing to cool is performed.
In the winding step, it is preferable to wind at a winding temperature of 450 to 650 ° C. from the viewpoint of the steel sheet structure. If the winding temperature is lower than 450 ° C, a large amount of martensite may be generated, and the total volume ratio of ferrite and bainite may be less than 70%. Further, the volume ratio of ferrite may be 30% or less. If the coiling temperature exceeds 650 ° C., the nucleation frequency of ferrite decreases, the ferrite grains coarsen, and the bainite transformation start temperature is exceeded, so that the volume ratio of bainite may not be 10% or more. The coiling temperature is more preferably 480 to 620 ° C, and even more preferably 490 to 590 ° C.

  As described above, the hot rolled steel sheet of the present invention is manufactured. According to the present invention, a hot-rolled steel sheet having a yield strength of 330 MPa or more, a tensile strength of 520 MPa or more, a yield ratio of 0.75 or less, and a Charpy absorbed energy at -40 ° C of 170 J or more can be obtained. Further, the cold roll-formed rectangular steel pipe manufactured using the obtained hot-rolled steel sheet as a raw material has a yield strength of 385 MPa or more and a tensile strength of 520 MPa or more, and can be provided with high plastic deformability and excellent toughness. As a result, it is possible to manufacture a high-strength rectangular steel pipe with high productivity and a short delivery time as compared with cold press bending. Since this cold roll-formed rectangular steel pipe can be suitably used particularly for building members of large-scale buildings such as factories, warehouses, and commercial facilities, it can greatly contribute to the reduction of construction cost.

  Hereinafter, the present invention will be described in more detail based on examples. The present invention is not limited to the examples below.

  Molten steel having the chemical composition shown in Table 1 was melted in a converter and made into a slab (steel material: wall thickness 250 mm) by a continuous casting method. The slab thus obtained was subjected to a hot rolling step, a cooling step and a winding step under the conditions shown in Table 2 to obtain a hot rolled steel sheet having a finished sheet thickness (mm) shown in Table 2.

  Test pieces were taken from the obtained hot rolled steel sheet, and the following structure observation, tensile test and Charpy impact test were carried out.

[Tissue observation]
The test piece for observing the structure was prepared so that the observation surface had a section in the rolling direction during hot rolling and had a plate thickness of 1 / 2t, and was polished and then corroded by nital. For the structure observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) was used to observe and image the structure at the plate thickness 1 / 2t position. From the obtained optical microscope image and SEM image, the area ratios of ferrite, pearlite, bainite and the remaining structure were determined. The area ratio of each tissue was calculated by observing in 5 or more visual fields and averaging the values obtained in each visual field. Here, the area ratio obtained by the structure observation was defined as the volume ratio of each structure.

  Here, ferrite is a product of diffusion transformation, and has a structure with a low dislocation density and almost recovered. This includes polygonal ferrites and pseudopolygonal ferrites. Bainite is a multiphase structure of lath-like ferrite and cementite with high dislocation density.

  Since it is difficult to distinguish between martensite and austenite in the optical microscope image and the SEM image, the area ratio of the structure observed as martensite or austenite was measured from the obtained SEM image, and then the austenite of the austenite measured by the method described below was measured. The value obtained by subtracting the volume ratio was taken as the volume ratio of martensite.

  The volume ratio of austenite was measured by X-ray diffraction. The test piece for observing the structure was produced by grinding so that the diffractive surface was located at the plate thickness 1 / 2t position of the steel sheet, and then chemical polishing to remove the surface processing layer. The Kα ray of Mo was used for the measurement, and the volume ratio of austenite was calculated from the integrated intensity of the (200), (220), (311) planes of fcc iron and the (200), (211) planes of bcc iron.

In addition, the volume ratio of the average circle equivalent diameter (average crystal grain size) and the crystal grains having a circle equivalent diameter (crystal grain size) of 40.0 μm or more was measured using the SEM / EBSD method. The measurement area was 500 μm × 500 μm, and the measurement step size was 0.5 μm. Regarding the crystal grain size, the orientation difference between adjacent crystal grains was obtained, and the boundary where the orientation difference was 15 ° or more was measured as the grain boundary. From the obtained crystal grain boundaries, the arithmetic average of the grain diameters was calculated to obtain the average crystal grain diameter.
In the crystal grain size analysis, those having a crystal grain size of 2.0 μm or less were excluded from the analysis target as measurement noise, and the obtained area ratio was made equal to the volume ratio.

[Tensile test]
The tensile test was carried out in accordance with JIS Z 2241 by taking a JIS 5 tensile test piece so that the tensile direction was parallel to the rolling direction. The yield strength YS and the tensile strength TS were measured, and the yield ratio defined by (yield strength) / (tensile strength) was calculated.

[Charpy impact test]
In the Charpy impact test, a V-notch test piece was sampled from the sheet thickness 1 / 2t position of the obtained hot-rolled steel sheet such that the longitudinal direction of the test piece was parallel to the rolling direction. According to JIS Z 2242, a Charpy impact test was conducted at a test temperature of -40 ° C, and the absorbed energy (J) was obtained. The number of test pieces was 3, and the average value was calculated to obtain the absorbed energy (J).

  The results obtained are shown in Table 3.

  In Table 3, Steel No. Steel Nos. 1, 4, 11, 12, 16, 21, 28, 30-40, 42 are examples of the present invention. 2, 3, 5 to 10, 13 to 15, 17 to 20, 29, 41 and 43 are comparative examples.

  The steel structures of the examples of the present invention each include ferrite with a volume ratio of more than 30%, 10% or more of bainite, the total volume ratio of ferrite and bainite is 70% or more and 95% or less, and the balance is pearlite or martens. Made of one or more selected from site and austenite, the average grain size of the crystal grains is less than 7.0 μm, and the volume percentage of the crystal grains having a grain size of 40.0 μm or more is 30%. It was below. As for the mechanical properties of these examples of the present invention, the yield strength was 330 MPa or more, the tensile strength was 520 MPa or more, the yield ratio was 0.75 or less, and the Charpy absorbed energy at -40 ° C was 170 J or more.

  On the other hand, the steel No. of the comparative example. In No. 2, since the C content was below the range of the present invention, the yield strength and tensile strength were out of the range of the present invention. Steel No. of the comparative example. In No. 3, since the Mn content was below the range of the present invention, the crystal grains became coarse, and the average grain size and the volume percentage of the crystal grains having a grain size of 40.0 μm or more were out of the range of the present invention. Therefore, the yield strength, the tensile strength, and the Charpy absorbed energy at −40 ° C. did not reach the desired values.

  Steel No. of the comparative example. In No. 5, the slab heating temperature was above the range of the present invention, the crystal grains were coarsened, and the average crystal grain size and the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more were out of the range of the present invention. The tensile strength and the Charpy absorbed energy at -40 ° C did not reach the desired values.

  Steel No. of the comparative example. In No. 6, the rolling reduction at 930 ° C. or lower is below the range of the present invention, it is not possible to suppress the formation of coarse bainite, and the volume ratio of crystal grains having a crystal grain size of 40.0 μm or more is outside the range of the present invention. Therefore, the Charpy absorbed energy at −40 ° C. did not reach the desired value.

  Steel No. of the comparative example. In No. 7, the finish rolling finish temperature was lower than the range of the present invention, and ferrite was generated during hot rolling, so that the yield ratio did not reach the desired value.

  Steel No. of the comparative example. In No. 8, since the finish rolling finish temperature exceeded the range of the present invention, the rolling reduction at 930 ° C. or less was less than the range of the present invention, the formation of coarse bainite could not be suppressed, and the crystal grain size of 40.0 μm or more. The volume ratio was outside the range of the present invention, and the Charpy absorbed energy at -40 ° C did not reach the desired value.

  Steel No. of the comparative example. In No. 9, since the cooling rate was below the range of the present invention, the crystal grains were coarsened, and the average crystal grain size and the volume ratio of the crystal grains having a crystal grain size of 40.0 μm or more were out of the range of the present invention. The strength and the Charpy absorbed energy at −40 ° C. did not reach the desired values.

  Steel No. of the comparative example. In No. 10, since the cooling rate exceeded the range of the present invention, the volume ratio of ferrite and the total volume ratio of ferrite and bainite were out of the range of the present invention, and the yield ratio and the Charpy absorbed energy at −40 ° C. reached desired values. Did not reach.

  Steel No. of the comparative example. In No. 13, the cooling stop temperature exceeded the range of the present invention, so the volume ratio of bainite was outside the range of the present invention, and the yield strength and tensile strength did not reach the desired values.

  Steel No. of the comparative example. In No. 14, since the cooling stop temperature was below the range of the present invention, the total volume fraction of ferrite and bainite was outside the range of the present invention, and the yield ratio and the Charpy absorbed energy at -40 ° C did not reach the desired values.

  Steel No. of the comparative example. In No. 15, since the content of C was above the range of the present invention, the total volume fraction of ferrite and bainite was outside the range of the present invention, and the yield ratio and the Charpy absorbed energy at -40 ° C did not reach the desired values. It was

  Steel No. of the comparative example. In No. 17, since the Si content exceeded the range of the present invention, the Charpy absorbed energy at -40 ° C did not reach the desired value.

  Steel No. of the comparative example. In No. 18, the yield ratio did not reach the desired value because the Mn content exceeded the range of the present invention.

  Steel No. of the comparative example. In No. 19, since the P content was above the range of the present invention, the Charpy absorbed energy at -40 ° C did not reach the desired value.

  Steel No. of the comparative example. In No. 20, since the S content was above the range of the present invention, the Charpy absorbed energy at -40 ° C did not reach the desired value.

  No. of the comparative example. In No. 29, the content of C was below the range of the present invention, so the yield strength and the tensile strength were out of the range of the present invention. Further, generation of pearlite which is a hard phase was suppressed, and the total volume ratio of ferrite and bainite was out of the range of the present invention. As a result, the yield ratio did not reach the desired value.

  No. of the comparative example. In No. 41, the cooling stop temperature was below the range of the present invention, so the volume ratio of ferrite was outside the range of the present invention, and the yield ratio did not reach the desired value.

  No. of the comparative example. In No. 43, since the cooling rate was below the range of the present invention, the average crystal grain size was outside the range of the present invention, and the Charpy absorbed energy at -40 ° C did not reach the desired value.

  From the above, by making the composition and structure of the hot rolled steel sheet within the scope of the present invention, used for building members of large-scale buildings, excellent in toughness, high strength, for low yield ratio cold roll forming square steel pipe A hot rolled steel sheet can be provided.

Claims (4)

The composition of components is% by mass,
C: 0.04% or more and 0.50% or less,
Si: 2.0% or less,
Mn: 0.5% to 3.0%,
P: 0.10% or less,
S: 0.05% or less,
Al: 0.005% or more and 0.10% or less,
N: 0.010% or less is contained, and the balance is Fe and inevitable impurities,
The steel structure at the 1 / 2t position of the plate thickness t from the surface of the steel plate is
In volume ratio, ferrite is more than 30%, bainite is 10% or more,
The total of the ferrite and the bainite is 70% or more and 95% or less with respect to the entire steel structure at the 1 / 2t position,
The balance consists of one or more selected from pearlite, martensite and austenite,
When the region surrounded by the boundary where the misorientation of adjacent crystals is 15 ° or more is a crystal grain,
The average equivalent circular diameter of the crystal grains is less than 7.0 μm,
Further, the hot-rolled steel sheet, wherein the total of the crystal grains having a circle equivalent diameter of 40.0 μm or more is 30% or less in terms of volume ratio with respect to the entire steel structure at the 1 / 2t position. The hot-rolled steel sheet according to claim 1, further comprising, in mass%, 1 group or 2 groups selected from the following A group and B group in addition to the component composition.
Note One or more selected from A group: Nb: 0.15% or less, Ti: 0.15% or less, V: 0.15% or less, B group: Cr: 1.0% or less, One or two selected from Mo: 1.0% or less, Cu: 0.5% or less, Ni: 0.3% or less, Ca: 0.010% or less, B: 0.010% or less. that's all   The hot-rolled steel sheet according to claim 1 or 2, having a thickness of more than 20 mm. It is a manufacturing method of the hot-rolled steel sheet according to any one of claims 1 to 3,
After heating the steel material having the above component composition to a heating temperature of 1100 ° C. or higher and 1300 ° C. or lower,
Rough rolling end temperature: 850 ° C. or more and 1150 ° C. or less, finish rolling end temperature: 750 ° C. or more and 850 ° C. or less, and total rolling reduction at 930 ° C. or less: 65% or more, hot rolling is performed,
After the hot rolling, a method for producing a hot-rolled steel sheet, comprising cooling at an average cooling rate at a sheet thickness center temperature: 10 ° C / s or more and 30 ° C / s or less and a cooling stop temperature: 450 ° C or more and 650 ° C or less.